1. Field of the Invention
The present invention relates to an analog to digital converting circuit, and more particularly, to a flash type analog to digital converting method and circuit.
2. Description of the Related Art
A conventional flash type analog to digital (A/D) converting circuit includes a pre-amplifier, a thermometer code generating circuit, and an encoder. The pre-amplifier and the thermometer code generating circuit receive an analog signal and generate a digital signal having a 2n-bit thermometer code. The encoder then directly converts the digital signal having the generated thermometer code into an n-bit digital signal. A thermometer code refers to a digital signal in which a bit string of “0”s and a bit string of “1”s are arranged to face each other, such as “000 . . . 000111 . . . 111”.
However, the conventional flash type A/D converting circuit has a disadvantage in that, since the encoder directly converts the digital signal having a 2n-bit of thermometer code into an n-bit of digital signal, a layout area of the encoder must be increased as the number of bits of the digital thermometer code is increased.
FIG. 1 is a block diagram of a conventional flash type A/D converting circuit. As shown in FIG. 1, the conventional flash type A/D converting circuit includes a pre-amplifier 10, a thermometer code generating circuit 12, and an encoder 14.
The pre-amplifier 10 amplifies an input analog signal Ain to generate K-number of analog signals Ain. The thermometer code generating circuit 12 receives the k-number of analog signals Ain to generate a digital signal having a 2n-bit thermometer code. The encoder 14 encodes the digital signal having a 2n-bit thermometer code to generate an n-bit digital signal Dout. The conventional flash type A/D converting circuit of FIG. 1 is configured such that the encoder 14 directly encodes the digital signal having a 2n-bit thermometer code to generate an n-bit digital signal Dout.
As a result, the layout area of the conventional flash type A/D converting circuit must be increased when the number of bits of the 2n-bit thermometer code generated from the thermometer code generating circuit 12 is large.
For example, when converting a 128-bit thermometer code into a 7-bit digital signal, the layout area of the converting circuit must be four times the layout area of a converting circuit used to convert a 32-bit thermometer code into a 5-bit digital signal.
That is, the layout area of the encoder 14 does not matter when the number of bits of the 2n-bit thermometer code is small. However, the layout area size of the encoder 14 does matter when the number of bits of the2n-bit thermometer code is large.